1. Field of the Invention
The present invention relates generally to the field of surge protection, and more particularly to a radio frequency (rf) surge protection device.
2. Description of the Related Art
Surge protection devices protect electronic equipment from being damaged by large variations in the current and voltage across power and transmission lines resulting from lightning strikes, switching surges, transients, noise, incorrect connections, and other abnormal conditions or malfunctions. Large variations in the power and transmission line currents and voltages can change the operating frequency range of the electronic equipment and can severely damage and/or destroy the electronic equipment. For example, lightning is a complex electromagnetic energy source having potentials estimated at from 5 million to 20 million volts and currents reaching thousands of amperes that can severely damage and/or destroy the electronic equipment.
Surge protection devices typically found in the art and used in protecting electronic equipment include capacitors, gas tubes, and metal oxide varistors (MOVs). A capacitor blocks the flow of direct current (dc) and permits the flow of alternating current (ac) depending on the capacitor""s capacitance and the current frequency. At certain frequencies, the capacitor might attenuate the ac signal. For example, the larger the capacitance value, the greater the attenuation. Typically, the capacitor is placed in-line with the power or transmission line to block the dc signal and undesirable surge transients.
Gas tubes contain hermetically sealed electrodes, which ionize gas during use. When the gas is ionized, the gas tube becomes conductive and the breakdown voltage is lowered. The breakdown voltage varies and is dependent upon the rise time of the surge. Therefore, depending on the surge, several microseconds may elapse before the gas tube becomes ionized, thus resulting in the leading portion of the surge passing to the capacitor. Gas tubes are attached at one end to the power or transmission line and at another end to the ground plane, diverting the surge current to ground.
MOVs are typically utilized as voltage limiting elements. If the voltage at the MOV is below its clamping or switching voltage, the MOV exhibits a high resistance. If the voltage at the MOV is above its clamping or switching voltage, the MOV exhibits a low resistance. Hence, MOVs are sometimes referred to as non-linear resistors because of their nonlinear current-voltage relationship. The MOV is attached at one end to the power or transmission line and at another end to the ground plane.
One drawback of conventional surge protection devices is the difficulty in impedance matching the surge protection device with the system. Another drawback of conventional surge protection devices is the elevated voltage at which they become conductive and the higher throughput energy levels.
One embodiment of the present invention is a surge protection device, which includes an input path for receiving an rf signal, dc power, and a surge, an output path for propagating the rf signal, and a dc blocking device coupled in series between the input path and the output path. The surge protection device also includes a first inductor coupled to the input path for isolating the rf signal and providing a path for the dc power and the surge, a gas tube coupled to the first inductor for routing a portion of the surge to a ground plane, a second inductor coupled to the first inductor for providing a path for the dc power, and a metal oxide varistor coupled to the second inductor for routing a portion of the surge to the ground plane. Furthermore, the surge protection device includes a third inductor coupled to the second inductor for providing a path for the dc power, a diode coupled to the third inductor for routing a portion of the surge to the ground plane, and a fourth inductor coupled to the third inductor for providing a path for the dc power to the output path. The diode conducts prior to the MOV, which conducts prior to the gas tube. Therefore, the diode diverts a first portion of the surge, the MOV diverts a second portion of the surge, and the gas tube diverts a third portion of the surge to the common ground. In one embodiment, the diode responds in nanoseconds, the MOV a short time thereafter, and the gas tube is the last element to respond to the surge. This sequence prevents most of the surge from reaching the output path.
Another embodiment of the present invention is an apparatus for isolating dc power and a surge from an rf path to improve the bandwidth of an rf signal that travels along the rf path. The apparatus includes a conductive plate, an inductor positioned adjacent to the conductive plate for routing the dc power and the surge away from the rf path, and means, coupled to the inductor, for diverting the surge to the conductive plate. The apparatus also includes a dc path coupled to the inductor for routing the dc power to the rf path.
Advantages of the surge protection device include dc circuitry on a plate or circuit board for passing dc currents, isolation from the rf signal path with inductors calculated to be high impedance to the respective rf bandwidth, and a unique cavity, which provides for an improved rf signal path and better impedance matching of the surge protection device and the system as compared to the more conventional rectangular cavity.
For purposes of summarizing the present invention, certain aspects, advantages, and novel features of the present invention have been described herein. Of course, it is understood that not necessarily all such aspects, advantages or features will be embodied in any one particular embodiment of the present invention.